First, an example of a conventional semiconductor potential supply device used in a semiconductor memory apparatus is hereinafter described.
FIG. 14 shows a semiconductor potential supply device (a step-down circuit) used for supplying a potential within a specific range to a conventional static random access memory device (SRAM) (see Japanese Laid-open Patent Publication, unexamined, Hei. 3207091). FIG. 15 is a diagram showing a relation between an external supply voltage and an internal supply voltage determined by this step-down circuit.
In FIG. 14, the step-down circuit includes an external power line VCC (potential VCC), a ground VG, resistors R1, R2, R3, P-channel MOS transistors Q1, Q2, an N-channel MOS transistor Q3, nodes N1, N2, N3, and an internal circuit C that is a semiconductor memory circuit such as a SRAM.
Operation of this conventional step-down circuit is described below. When the external supply voltage is low, for example, 3 V, the P-channel MOS transistor Q1 is turned off by the voltage of the node N1 determined by the ratio of resistors R1 and R2, and the node N2 is lowered nearly to 0 V by the resistor R3. As a result, the P-channel MOS transistor Q2 is turned on, and the voltage VINT of the node N3 supplied to the internal circuit C becomes 3 V that is the same as the external voltage VCC.
On the other hand, when the external voltage VCC becomes higher than a prescribed value for activity, or "action point", for example 5 V, the P-channel MOS transistor Q1 is turned on by the node N1, and the node N2 is raised nearly to the supply voltage VCC, and the P-channel MOS transistor Q2 is turned off. As a result, current through the N-channel MOS transistor Q3 is supplied to the internal circuit C, and its internal supply voltage VINT is lowered from 5 V of the external supply voltage VCC by a portion of the threshold voltage Vtn', i.e., about 1.5 V of the transistor Q3 having back gate effect, and drops to about 3.5 V.
In this way, reliability is secured for preventing high voltage from being applied to the internal circuit C, and at the same, even when the external voltage VCC is lowered, data of memory cells in the internal circuit C is not lost. The prescribed value (action point) to change whether the potential supplied to the internal circuit C is directly coupled to the external potential VCC or internally lowered in the step-down circuit is determined substantially by the ratio of the resistors R1 and R2.
FIG. 16 shows a relation between a potential at node N1 and an external voltage VCC in the conventional step-down circuit. Supposing that the external voltage is VCC, a potential at the node N1 is VN1, and a threshold voltage without back gate effect of the P-channel MOS transistor Q1 is Vtp, the P-channel MOS transistor Q1 is turned on when the following relation is established. EQU VCC-VN1=Vtp
And a step-down state and a direct coupling state are changed over. The VCC at this moment is called an action point Vsp. At this time, following expressions are obtained. EQU Vsp-VN1=Vtp EQU Vsp=VN1+Vtp
In the conventional step-down circuit, however, the potential of the node N1 is determined by the ratio of resistors R1:R2. If this ratio deviates or fluctuates due to variation of manufacturing process or the like, the prescribed action point value may be shifted out of the intended voltage.
In FIG. 16, line "b" represents magnitude of external supply voltage, and line "c" shows a potential at node N1 at a specific resistor ratio R1:R2. Therefore, when VCC-VN1 is Vtp, the desired action point value VSP is achieved. However, when the resistor ratio R1:R2 is varied, the potential of the node N1 becomes as indicated by line "d" for example, and the prescribed value becomes varied.
In this manner, in the conventional step-down circuit, there arises the above-discussed problem and, moreover, the following problem takes place. First of all, particularly in a SRAM of low power consumption, the resistors used in the step-down circuit are as high as a several hundred megohm level in order to decrease the current consumed in the step-down circuit itself. These resistors are composed of very thin films in order to have a large resistor value. And since the films are thin, the resistor value thereof fluctuates significantly depending on formation of crystal, azimuth or bearing, etc. Therefore, a problem exists in that even if the voltage is subject to resistor division, for example, divided into 1:4, it may not actually be always 1:4.
Moreover, since the resistor value is very large as mentioned above, current is very small, and when the supply voltage is 5 V, for example, current is about 50 nA (5.times.10-8 A) at most. Therefore, the node N1 whose potential is determined by the resistor division requires a very long time until a desired potential is established. Hence, being unable to track sudden changes of external voltage VCC, the step-down circuit may supply an unintended voltage until the potential of the node N1 is sufficiently established.